162 BOTANY FOR BEGINNERS CHAP. 



The change from the diurnal to the nocturnal position, which 

 many leaves undergo, protects them from rain, snow, hail, 

 changes in temperature, and prevents loss of heat by radiation. 

 The leaves are folded so as to expose the minimum amount of 

 surface to the air during their nocturnal position, and during the 

 diurnal as much surface as possible is exposed for assimilation. 



SUMMARY. 



Growth means a permanent change of form. It is only living things 

 which grow. 



Conditions necessary for Growth. ( i ) A certain temperature which 

 varies for different plants. For every plant there is a minimum tem- 

 perature below which no growth can take place, and a maximum above 

 which growth will be arrested. Between these points the optimum 

 temperature occurs. 



(2) Moisttire must be present, because it enters into the composition 

 of the protoplasm. 



(3) Oxygen is necessary for most plants. 



(4) Suitable food materials must be present. 



(5) Light is necessary for the healthy growth of all green plants. 

 Light prevents too rapid growth. 



Irritability means the property of protoplasm to respond to external 

 influences. Most growing and some mature organs possess this property. 

 Light, gravitation, and moisture are the principal agents which produce 

 movements in growing plants. 



The Action of Light on Growing Organs. Heliotropism refers to 

 the power of turning either towards the light or away from it which 

 plants possess. Most shoots bend towards the light and are said to be 

 positively heliotropic. Roots, rhizomes, and bulbs turn from the light 

 and are said to be negatively heliotropic. 



Heliotropic movements take place as a result of changes in the 

 length of one side of the organ in comparison to the other. This 

 produces curvature. 



Geotropism is the property which enables the organs of plants to 

 take up a definite position in regard to gravitation. 



Movements caused by Contact. When a tendril comes in contact 

 with a support, the side which touches it has its growth arrested, and 

 the opposite side grows more quickly. This causes curvature, and 

 enables the tendril to twine round the support. 



The Sleep of Flowers and Leaves. Many flowers and leaves change 

 their position from day to night. The movements are produced by 

 changes in the amount of water which the cells of the various parts of 

 foliage leaves or floral leaves contain. 



The Utility of the Movements. All the movements which the various 

 parts of plants perform are to bring the plants into touch with their 

 surroundings. 



